Organic silicon oxide fine particles and preparation method thereof, porous film-forming composition, porous film and formation method thereof, and semiconductor device

ABSTRACT

Provided are organic silicon oxide fine particles which can be formed into a porous film having a dielectric constant and mechanical strength expected as a high-performance porous insulating film and having excellent chemical stability, and a preparation method thereof. Described specifically, provided are an organic silicon oxide fine particle comprising a core containing at least an inorganic silicon oxide or an organic silicon oxide and a shell containing at least an organic silicon oxide and being formed around the core by using shell-forming hydrolyzable silane in the presence of a basic catalyst; wherein of silicon atoms constituting the core or the shell, a ratio (T/Q) of a number (T) of silicon atoms having at least one bond directly attached to a carbon atom to a number (Q) of silicon atoms having all of four bonds attached to an oxygen atom is greater in the shell than in the core; and wherein the shell-forming hydrolyzable silane comprise at least a hydrolyzable silane compound having two or more hydrolyzable-group-having silicon atoms bound to each other via a carbon chain or via a carbon chain containing one silicon atom between some carbon atoms.

CROSS-RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.12/472,681, filed May 27, 2009, which claims priority from JapanesePatent Application No. 2008-142344; filed May 30, 2008, the disclosuresof which are incorporated herein by reference in their entireties.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to organic silicon oxide fine particleswhich can be formed into a porous film excellent in dielectricproperties, mechanical strength and chemical stability by application, apreparation method thereof, a film-forming composition, a formationmethod of a porous film, a porous film formed thereby, and asemiconductor device having the porous film.

2. Description of the Related Art

In the fabrication of semiconductor integrated circuits, as theirintegration degree becomes higher, an increase in interconnect delaytime due to an increase in interconnect capacitance, which is aparasitic capacitance between metal interconnects, prevents theirperformance enhancement. The interconnect delay time is called an RCdelay which is in proportion to the product of the electric resistanceof metal interconnects and the static capacitance between interconnects.Reduction in the resistance of metal interconnects or reduction in thecapacitance between interconnects is necessary for reducing thisinterconnect delay time. The reduction in the resistance of aninterconnect metal or interconnect capacitance can prevent even a highlyintegrated semiconductor device from causing an interconnect delay,which enables size reduction and high speed operation of it andmoreover, minimization of power consumption.

In order to reduce the resistance of metal interconnects, semiconductordevice structures using copper as metal interconnects have recentlyreplaced those using conventional interconnects made of aluminum. Use ofcopper interconnects alone, however, has limits in accomplishingperformance enhancement so that reduction in the interconnectcapacitance is an urgent necessity for further performance enhancementof semiconductor devices.

One method for reducing interconnect capacitance is to reduce thedielectric constant of an interlayer insulating film disposed betweenmetal interconnects. As such a low dielectric constant insulating film,use of a porous film instead of a conventionally used silicon oxide filmis now studied. In particular, since a porous film is only one practicalfilm as a material being suited as an interlayer insulating film andhaving a dielectric constant not greater than 2.5, various methods forforming a porous film have been proposed. When an interlayer insulatingfilm is made porous, however, reduction in mechanical strength andadsorption of water are likely to deteriorate the film so that reductionin dielectric constant (k) by introduction of pores into the film andmaintenance of sufficient mechanical strength and hydrophobicity areserious problems that need to be overcome.

A silica film having enhanced mechanical strength can be obtained, forexample, by increasing the proportion of tetrafunctional silicon unitsas a silicon unit constituting the film, thereby constructing a denselycrosslinked siloxane structure to form hard particles. In practice, afilm obtained by plasma polymerization of tetrafunctional TEOS showsstrength as high as 80 GPa in bulk form (form having no porosity). Whena film is prepared from a hydrolysis condensate of a trifunctionalalkoxysilane having a methyl group, on the other hand, it shows strengthof 20 GPa or less even in bulk form (“Low-k Materials and ProcessIntegration after the 65 nm and 45 nm Generations”, by Eiki Shibata,proceedings of a lecture held by Electronic Journal on Apr. 18, 2006, atOchanomizu/Tokyo). Even if pores are introduced into the above film todecrease their dielectric constant, the strength in bulk form stillmaintains. Accordingly, it is known that as the proportion oftetrafunctional units becomes larger, high strength can be achieved moreeasily.

With regard to chemical properties, the binding energy itself of a Si—Obond is greater than that of a Si—C bond so that the former gives astructure resistant to heat decomposition. Difference in reactivity witha chemical substance such as washing fluid is, on the other hand,attributable to a large difference in polarity between the Si—C bond andthe Si—O bond. The Si—O bond having a greater polarity is susceptible tothe attack (nucleophilic attack) of the chemical substance. Similarly,comparison in polarity between tetrafunctional silicon and trifunctionalsilicon has revealed that an electron density at the center oftetrafunctional silicon lowers (greater δ+) with the number of Si—Obonds having a large polarity so that it is susceptible to nucleophilicattack. When the number of Si—O bonds decreases as silicon becomestrifunctional or bifunctional, the electron density at the center of thesilicon shows a small decrease (smaller δ+). As a result, it is notsusceptible to the nucleophilic attack.

When a porous silica film is used as an interlayer insulating film of asemiconductor device, process damage in an etching or washing step posesa problem. In particular, hydrophilization of the surface of the poroussilica film after treatment with a washing fluid and moisture absorptionresulting therefrom lead to deterioration in the reliability of thesemiconductor device. There is therefore a demand for overcoming such aproblem.

It has been recognized that the susceptibility of a CVD-LK film to sucha process damage becomes smaller with an increase in its carbon content.Also in an LK film of an application type, an increase in carbon contentby introducing a carbosilane skeleton is under study (JP 2007-262257A).

SUMMARY OF THE INVENTION

An object of the invention is to provide organic silicon oxide fineparticles which can be formed into a porous film satisfying an expecteddielectric constant and mechanical strength and having excellentchemical stability by using a silica sol as an industrially desirablematerial in order to obtain a high-performance porous insulating film byapplication, and a preparation method of the organic silicon oxide fineparticles, a film-forming composition containing them, a preparationmethod of a porous film, and a porous film formed thereby.

Another object of the invention is to provide a high performance andhigh reliability semiconductor device having the porous film obtainedusing the advantageous material.

As described above, when a film is viewed as a whole, there is atrade-off relationship between maintenance of mechanical strength andimprovement in chemical stability by incorporating a substituent, suchas alkyl or alkylene, containing carbon having a direct bond to siliconin a hydrolyzable silane compound used for obtaining silica to be usedas a film material, thereby increasing a ratio (T/Q ratio) of the number(T) of silicon atoms having a bond directly attached to a carbon atom tothe number (Q) of silicon atoms having four bonds all of which areattached to an oxygen atom. Simple blending of a material having highmechanical strength and a material having high chemical stabilityresults in the formation of the corresponding material which is not anexpected material.

The present inventors therefore made the following working hypothesisfor improving the performance of a porous-film-forming coating solutionmaking use of silica.

According to their hypothesis, it is preferred to place parts havingrespective functions only at required positions thereof in order toobtain physical properties different among the positions; and moreover,it is preferred to use a material in which only necessary amounts ofpotentially necessary parts are arranged at proper positions in order toachieve such controlled arrangement by using a uniform coating solution.It is possible to achieve such a particular arrangement by employing astructure in which a core portion of silica particles and an peripheralfilm covering the periphery of the core portion are derived fromdifferent materials, respectively. A film in which a materialconstituting a core portion and a material constituting an peripheralfilm have been arranged regularly can be obtained only by applying acoating solution of such organic silicon oxide fine particles to asubstrate. Composite type organic silicon oxide fine particles usingdifferent materials for core and shell, respectively, are thus presumedto be useful.

Further, the present inventors thought that a film formed usingcomposite type organic silicon oxide fine particles obtained using amaterial having high mechanical strength for the core and anothermaterial capable of giving chemical stability for the shell has highchemical stability because the above T/Q ratio in a region contiguous tothe outside is high and at the same time, cores are arranged atintervals formed by the shell to achieve high mechanical strength whilepreventing uneven presence of the material having low mechanicalstrength. Moreover, the present inventors thought that when the shell issoft, a contact area of the organic silicon oxide fine particles eachother becomes wide, interparticle bonds are formed by baking whilemaintaining the wide contact area, and formation of a matrix having highmechanical strength can be expected.

In the surface modification for changing the quality of silica particlesor zeolite particles, a method of modifying the side chain thereofhaving a mercapto group in order to give a bond formation capacity to apolymerizable functional group is known (JP 10-81839A). This methodgives reactivity while offering freedom to the surface-modifiedfunctional group. Since an increase in condensation degree is notpreferable for silane having a substituent, surface modification in JP10-81839A is performed in the presence of an acid catalyst. From thestandpoint of preventing silicon from undergoing nucleophilic attack inorder to overcome the problem of the invention, the peripheral film isrequired to be crosslinked densely and thereby have a function ofpreventing invasion of a nucleophilic species into the inside of theparticles. The particles obtained using an acid catalyst are thereforenot preferred.

The present inventors disclose a method of modifying organic siliconoxide fine particles with a crosslinkable side chain in the presence ofa basic catalyst, thereby improving an interparticle bonding power (JP2005-216895A). This method uses a basic catalyst for freezing theactivity of the crosslinking group, but it does not include a concept ofimparting chemical stability to the particles by surface modification.

The present inventors have carried out an intensive investigation basedon the above hypothesis. As a result, they have succeeded in forming aporous film having both mechanical strength and chemical stability byusing a porous film-forming composition containing composite type silicafine particles. The composite type silica fine particles are obtained byforming a core of organic silicon oxide fine particles from a materialmainly containing a tetravalent hydrolyzable silane in the presence of abasic catalyst and then by forming a shell, so as to cover the peripheryof the core, by using an organic silicon oxide which has a unit havingsilicon atoms bonded via a hydrocarbon crosslink and mainly comprisessilicon atoms each having a substituent having a carbon atom attacheddirectly to a silicon atom. Moreover, they have found a preparationmethod of a coating composition capable of providing a film havingimproved physical properties suited for use even in a semiconductorfabrication process, leading to the completion of the invention. In thistechnology, not only inorganic or organic silica fine particles but alsozeolite fine particles can be used as the core material. Use of themenables to enhance the strength of the core further.

In one aspect of the invention, there is thus provided an organicsilicon oxide fine particle comprising:

a core containing at least an inorganic silicon oxide or an organicsilicon oxide and

a shell containing at least an organic silicon oxide and being formedaround the core by using shell-forming hydrolyzable silane in thepresence of a basic catalyst;

wherein of silicon atoms constituting the core and shell, a ratio (T/Q)of a number (T) of the silicon atoms having at least one bond directlyattached to a carbon atom to a number (O) of silicon atoms having all ofthe four bonds attached to an oxygen atom is greater in the shell thanin the core; and

wherein the shell-forming hydrolyzable silane comprise at least ahydrolyzable silane compound having two or morehydrolyzable-group-having silicon atoms bound to each other via a carbonchain or via a carbon chain containing one silicon atom between somecarbon atoms.

In the composite type organic silicon oxide fine particle of theinvention, the core has a smaller T/Q ratio than the shell so that ithas a high Si—O—Si bond density and therefore has high mechanicalstability. The shell, on the other hand, has a greater T/Q ratio thanthe core and has a skeleton providing a dense crosslink density so thatthe composite type organic silicon oxide fine particle can have ahydrophobic skin with a high condensation degree in spite of an increasein the T/Q ratio and therefore have chemical stability against a washingfluid. The shell having a greater T/Q ratio than the core has highspatial freedom and deforms easily so that it serves to increase thespatial interaction area between particles in a film formed using them.

According to another mode of the organic silicon oxide fine particle ofthe invention, said shell forming hydrolyzable silane comprises one ormore compounds represented by the following formula (1):

{R¹ _(n)X¹ _(3-n)Si—[(Y²)—(SiR² _(m)X² _(2-m))]_(p)}_(q)—(Y³)—SiR³_(t)X³ _(3-t)  (1)

wherein X¹ to X³ each independently represents a hydrolyzable groupselected from the group consisting of a hydrogen atom, halogen atoms andC₁₋₄ alkoxy groups; R¹ to R³ each independently represents a C₁₋₂₀ alkylgroup or a C₆₋₁₀ aryl group; Y² and Y³ each independently represents asubstituted or unsubstituted C₁₋₆ hydrocarbon group having q+1valencies, a C₅₋₂₀ cycloalkane group which has q+1 valencies and maycontain a fused ring structure, or a C₆₋₂₀ aromatic group having q+1valencies; m each independently represents an integer from 0 to 2; neach independently represents an integer from 0 to 2; p eachindependently represents an integer from 0 to 4; q each independentlyrepresents an integer of 1 or greater, and t each independentlyrepresents an integer from 0 to 2.

According to a further mode of the organic silicon oxide fine particleof the invention, said one or more compounds represented by the formula(1) is selected from the group consisting of compounds represented bythe following formula (2):

and the following formula (3):

wherein X⁴ to X⁹ each independently represents a hydrolyzable groupselected from the group consisting of a hydrogen atom, halogen atoms andC₁₋₄ alkoxy groups; R⁴ to R⁹ each independently represents a C₁₋₂₀ alkylgroup or a C₆₋₁₀ aryl group; m each independently represents an integerfrom 0 to 2; n each independently represents an integer from 0 to 2; peach independently represents an integer from 0 to 4; r eachindependently represents an integer from 0 to 4; each independentlyrepresents an integer from 0 to 4; t each independently represents aninteger from 0 to 2; and u each independently represents an integer from0 to 4.

According to a still further mode of the organic silicon oxide fineparticle of the invention, the number of silicon atoms contained in thecore is greater than the number of silicon atoms contained in the shell.Since the number of silicon atoms contained in the core is greater thanthat in the shell, the fine particle can exhibit the mechanical strengthproperties of the core desirably.

According to a still further mode of the organic silicon oxide fineparticle of the invention, the core contains a zeolite-like recurringstructure. Although zeolite-like fine particles are outside thedefinition of zeolite because the particle size thereof is too small todiscuss its long-range regularity, zeolite itself and a recurringstructure which zeolite partially has are called collectively“zeolite-like recurring structure”. It has higher mechanical strengththan that of amorphous silicon oxides. An Organic silicon oxide fineparticle containing a core having this zeolite-like recurring structurecan therefore have higher mechanical strength.

According to a still further mode of the organic silicon oxide fineparticle of the invention, said inorganic silicon oxide or said organicsilicon oxide of said core is an inorganic or organic silica prepared byhydrolysis/condensation of a core-forming hydrolyzable silane in thepresence of a basic catalyst. The hydrolysis and condensation of ahydrolyzable silane can raise a Si—O —Si bond density when it isperformed in the presence of a basic catalyst and as a result, theorganic silicon oxide fine particle can have high mechanical strength.

According to a still further mode of the organic silicon oxide fineparticle of the invention, said shell-forming hydrolyzable silaneconsists essentially of one or more hydrolyzable silane compounds havinga carbon atom directly attached to a silicon atom. The term “consistessentially of” means that 95 mol % or greater, in terms of silicon (thenumber of silicon atoms), more preferably 98 mol % or greater, stillmore preferably 100% of the shell-forming hydrolyzable silane ishydrolyzable silane substituted with a substituent having a carbon atomdirectly attached to a silicon atom. This makes it possible to preventformation of a portion having weak chemical stability on the surface ofthe shell and impart high chemical stability to the whole fine particle.

According to a still further mode of the organic silicon oxide fineparticle of the invention, it comprises an intermediate layer betweenthe core and the shell. The silicon oxide fine particle may consistessentially of a core and a shell, but it may have an intermediate layertherebetween. The thickness of the shell should be increased slightlywhen the intermediate layer is inserted and this leads a slightreduction in the improving effect of mechanical strength derived fromthe core. But the intermediate layer can widen the contact area betweenparticles at the time of film formation so that a film obtained usingsuch silicon oxide fine particle can have chemical stability withoutreducing the mechanical strength of the film itself.

In another aspect of the invention, there is also provided a method forproducing an organic silicon oxide fine particle, comprising steps of:

adding first hydrolyzable silane to water or a mixed solution of waterand an alcohol to carry out hydrolysis and condensation of the resultingmixture in the presence of a basic catalyst to form a core,

wherein the first hydrolyzable silane is a silane compound or compounds,containing at least one compound represented by the following formula(4):

Si(OR¹⁰)₄  (4)

wherein R¹⁰ may be the same or different and each independentlyrepresents a linear or branched C₁₋₄ alkyl group; and

adding, to the reaction mixture for the core, second hydrolyzable silanewhich is a hydrolyzable silane compound or a mixture of two or morehydrolyzable silane compounds to form a shell,

wherein of silicon atoms constituting the first hydrolyzable silane orthe second hydrolyzable silane, a ratio (T/Q) of a number (T) of siliconatoms having at least one bond directly attached to a carbon atom to anumber (Q) of silicon atoms having all of the four bonds attached to anoxygen atom is greater in the second hydrolyzable silane than in thefirst hydrolyzable silane; and

the second hydrolyzable silane contains a hydrolyzable silane compoundhaving two or more hydrolyzable-group-having silicon atoms bound to eachother via a carbon chain or via a carbon chain containing one siliconatom between some carbon atoms. Use of the production method comprisingsuch operations facilitates production of silicon oxide fine particlehaving, on the periphery of a core with high mechanical stability, ashell with high chemical stability.

According to another aspect of the method for producing an organicsilicon oxide fine particle of the invention, after addition of a totalamount of the first hydrolyzable silane, reaction conditions permittingprogress of the hydrolysis and condensation of the added firsthydrolyzable silane are maintained and the step of adding of the secondhydrolyzable silane is started. Insertion of the so-called agingoperation as described above enables to form a shell with a thin layerand as a result, the mechanical strength of the core can be reflectedhighly in the particle.

According to a further aspect of the method for producing an organicsilicon oxide fine particle of the invention, prior to completion of theaddition of a total amount of the first hydrolyzable silane, the step ofadding of the second hydrolyzable silane is started. Use of such aprocess facilitates formation of an intermediate layer between the coreand the shell, having an intermediate composition therebetween, and asdescribed above, chemical stability can be imparted withoutsignificantly reducing the mechanical strength of the film itself.

According to a further aspect of the method for producing an organicsilicon oxide fine particle of the invention, the second hydrolyzablesilane is represented by the following formula (1):

{R¹ _(n)X¹ _(3-n)Si—[(Y²)—(SiR² _(m)X² _(2-m))]_(p)}_(q)—(Y³)—SiR³_(t)X³ _(3-t)  (1)

wherein, X¹ to X³ each independently represents a hydrolyzable groupselected from the group consisting of a hydrogen atom, halogen atoms andC₁₋₄ alkoxy groups; R¹ to R³ each independently represents a C₁₋₂₀ alkylgroup or a C₆₋₁₀ aryl group; Y² and Y³ each independently represents asubstituted or unsubstituted C₁₋₆ hydrocarbon group having q+1valencies, a C₅₋₂₀ cycloalkane group which has q+1 valencies and maycontain a fused ring structure, or a C₆₋₂₀ aromatic group having q+1valencies; m each independently represents an integer from 0 to 2, n(s)each independently represents an integer from 0 to 2; p eachindependently represents an integer from 0 to 4; q each independentlyrepresents an integer of 1 or greater; and t each independentlyrepresents an integer from 0 to 2.

In a further aspect of the invention, there is also provided aporous-film-forming composition containing the organic silicon oxidefine particle and an organic solvent. Use of the porous-film-formingcomposition facilitates production of a porous film having both highmechanical stability and high chemical stability.

In a still further aspect of the invention, there is also provided aporous film obtained using the porous-film-forming composition. Theporous film of the invention has high mechanical strength and at thesame time, high chemical stability so that it can be suited for usesrequiring to satisfy both of them simultaneously, particularly a lowdielectric constant film to be used in a semiconductor device.

In a still further aspect of the invention, there is also provided amethod for forming a porous film, comprising steps of:

applying the porous-film-forming composition to form a film, and

subjecting the film to heat and/or to an electron beam or light. By themethod comprising the step of applying the porous-film-formingcomposition to form a film and the heating step, a porous film havinghigh mechanical strength and high chemical stability can be obtained.

According to another mode of the method for forming a porous film of theinvention, said step of subjecting comprises subjecting to heat and toan electron beam or light. The film exposed to an electron beam or lighthas higher strength because it increases the number of Si—O—Si bondsefficiently.

In a still further aspect of the invention, there is also provided asemiconductor device comprising the porous film as an insulating film.The semiconductor device using the porous film as an insulating film inthe production process of it can have high reliability.

The invention makes it possible to provide an organic silicon oxide fineparticle which can be formed into a porous film excellent in dielectricproperties, mechanical strength, and chemical stability by application,a production method thereof, a film-forming composition, a formationmethod of a porous film and a porous film formed thereby, and asemiconductor device having the porous film.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention now will be described more fully hereinafter inwhich embodiments of the invention are provided with reference to theaccompanying drawings. This invention may, however, be embodied in manydifferent forms and should not be construed as limited to theembodiments set forth herein; rather, these embodiments are provided sothat this disclosure will be thorough and complete, and will fullyconvey the scope of the invention to those skilled in the art.

The terminology used in the description of the invention herein is forthe purpose of describing particular embodiments only and is notintended to be limiting of the invention. As used in the description ofthe invention and the appended claims, the singular forms “a”, “an” and“the” are intended to include the plural forms as well, unless thecontext clearly indicates otherwise.

Unless otherwise defined, all technical and scientific terms used hereinhave the same meaning as commonly understood by one of ordinary skill inthe art to which this invention belongs.

Hereinafter, preferred embodiments of the present invention will bedescribed. However, it is to be understood that the present invention isnot limited thereto.

The organic silicon oxide fine particles and production method thereof,film-forming composition, porous film and formation method thereof, andsemiconductor device according to the invention will hereinafter bedescribed specifically. The present invention is however not limited tothe following embodiments.

The present invention relates to organic silicon oxide fine particlescomprising a core containing at least an inorganic silicon oxide or anorganic silicon oxide and a shell containing at least an organic siliconoxide formed around the core by using a hydrolyzable silane in thepresence of a basic catalyst. They are composite type organic siliconoxide fine particles comprising a core which has a smaller T/Q ratiothan that of the shell, a high Si—O—Si density, and therefore hasexcellent mechanical strength, wherein the T/Q ratio means that, ofsilicon atoms constituting the fine particles, a ratio of the number (T)of silicon atoms having at least one bond directly attached to a carbonatom to the number (O) of silicon atoms having all the four bondsdirectly attached to an oxygen atom; and a hydrophobic skin having ahigher T/Q ratio than that of the core and having a skeleton derivedfrom multinuclear hydrolyzable silane having hydrolyzable-group-havingsilicon atoms bound to each other via a hydrocarbon and capable ofgiving a dense crosslink density and mechanical flexibilitysimultaneously, and therefore having a high condensation degree. Thecomposite-type organic silicon fine particles therefore have chemicalstability against a washing fluid or the like and have softness only onthe surface of them. An object of the organic silicon oxide fineparticles of the invention is to form a film having a micro regulararrangement by using the organic silicon oxide fine particles of theinvention, which use different materials for the core and the shellrespectively, and allow them to exhibit desirable physical properties,respectively, compared with use of these materials simply as a mixed orbonded material.

The organic silicon oxide fine particles found by the present inventorsand having both mechanical strength and chemical stability have alayered structure in which the hard core contributing to mechanicalstrength is covered completely with a shell contributing to chemicalstability and mechanical flexibility.

The organic silicon oxide fine particles of the invention have anaverage particle size of preferably 50 nm or less, more preferably 5 nmor less. The organic silicon oxide fine particles having a particle sizeexceeding 50 nm may generate striation upon spin coating and thus havean adverse effect. The particle size of the fine particles can bemeasured using, for example, a submicron particle size distributionanalyzer “N4Plus” (trade name; product of Coulter), but its lowermeasurement limit is 2 nm. There is no effective means for measuring theparticle sizes less than 2 nm. The preferable lower limit of theparticle size can therefore be considered theoretically as follows.Described specifically, the average particle size of the core less than0.5 nm is not preferred, because a proportion of a shell component whichwill be described later may become too high relative to the corecomponent, leading to shortage in physical strength for which the coremust be responsible. The thickness of the shell is preferably from 0.025to 0.5 nm, more preferably from 0.05 to 0.2 nm. The shell having athickness less than 0.025 nm may not sufficiently cover the surface ofthe particles and therefore may not achieve expected chemical stability.The thickness exceeding 0.5 nm, on the other hand, may presumably causelack of physical strength because the proportion of the shell componentmay become too high relative to the core component.

[Core]

An inorganic silicon oxide or an organic silicon oxide can be used forthe core having high mechanical strength. More specifically, materialsconventionally used as a constituent material of a porous-film-formingcomposition for imparting mechanical strength to a film such as siliconoxide fine particles having a zeolite-like recurring structure and aninorganic or organic silica can be used. (I) Core containing siliconoxide having a zeolite-like recurring structure

Silicon oxide having a zeolite-like recurring structure includes asdescribed above zeolite itself, clusters having a size of about 1 nm andhaving crystal lattices arranged with insufficient regularity, andzeolite crystal precursors having a size of from about 10 to 15 nm. Theywill hereinafter be called zeolite collectively and simply.High-strength organic silicon oxide fine particles can be obtainedusing, as a core, zeolite having markedly great mechanical strength.

Zeolite crystals can be obtained, for example, by mixingtetraethoxysilane and tetrapropylammonium hydroxide, reacting themixture at room temperature for 3 days or more to obtain a seed crystal,then reacting the resulting seed crystal at 80° C. for 10 hours. When anorganic-group-containing silane component is added duringhigh-temperature reaction, however, formation of zeolite crystals doesnot proceed completely. The formation process of zeolite crystals can beconfirmed by XRD. Compared with zeolite crystals obtained by theordinary reaction, those using a zeolite seed crystal have difficulty inexhibiting a clear analysis pattern because of insufficient crystalgrowth. Although the reaction product obtained by adding an organicsilane component has disorders in the crystal structure and includes anoise in its analysis pattern, signals derived from the crystalstructure can be observed.

Zeolite fine particles to be used for the core of the inventionpreferably have an average particle size of from 0.5 to 50 nm. Zeolitefine particles can be synthesized by the hydrothermal synthesis of asilane having, on the silicon atom thereof, four hydrolyzable groupssuch as tetraethoxysilane (which will hereinafter be called “Q unitprecursor” or “Q unit monomer”) and an ammonium salt called“structure-directing agent”. Use of zeolite fine particles synthesizedin a conventional manner and having a particle size exceeding 100 nm mayroughen the surface of a coated film. Zeolite fine particles can besynthesized advantageously by the hydrothermal synthesis at lowtemperatures as disclosed by the present inventors in JP 2004-161535A.

Zeolite fine particles can be obtained by hydrolyzing preferably asilane compound represented by the following formula (4):

Si(OR¹⁰)₄  (4)

wherein R¹⁰ may be the same or different and each independentlyrepresents a linear or branched C₁₋₄ alkyl group,in the presence of a structure-directing agent and a basic catalyst,followed by heating treatment. The agent and the catalyst will bedescribed later.

Examples of the preferred silane compound of the formula (4) to be usedfor the formation of zeolite fine particles include tetramethoxysilane,tetraethoxysilane, tetrapropoxysilane, tetrabutoxysilane,tetraisopropoxysilane, tetraisobutoxysilane, triethoxymethoxysilane,tripropoxymethoxysilane, tributoxymethoxysilane, trimethoxyethoxysilane,trimethoxypropoxysilane, and trimethoxybutoxysilane. These silanecompounds may be used either singly or in combination.

It is known that the structure-directing agent determines the crystaltype of zeolite and thus has an important role. The structure-directingagent may preferably include, for example, a quaternary organic ammoniumhydroxide represented by the following formula (5):

(R¹¹)₄N⁺OH⁻  (5)

wherein R¹¹ may be the same or different and each represents a linear orbranched C₁₋₅ alkyl group.

Specific preferred examples of R¹¹ include methyl, ethyl, propyl andbutyl groups. Specific examples of such a structure-directing agentinclude tetramethylammonium hydroxide, tetraethylammonium hydroxide,tetrapropylammonium hydroxide, tetrabutylammonium hydroxide,triethylmethylammonium hydroxide, tripropylmethylammonium hydroxide andtributylmethylammonium hydroxide.

For the preparation of a zeolite sol, the structure-directing agent maybe used as a mixture with a silane compound. The structure-directingagent is added in an amount of preferably from 0.1 to 20 mols, morepreferably from 0.5 to 10 mols per mol of the silane compound orcompounds represented by the formula (4).

The basic catalyst used in the synthesis may serve to acceleratehydrolysis and condensation of the silane compound.

Preferred examples of the basic catalyst include compounds representedby the following formula (6):

(R¹²)₃N  (6)

wherein R¹² may be the same or different and each independentlyrepresents a hydrogen atom or a linear, branched or cyclic C₁₋₂₀ alkylor aryl group, with the proviso that the hydrogen atom contained in thealkyl or aryl group may be substituted with a hydroxy or amino group;and compounds represented by the following formula (7):

(R¹³)_(p)X¹⁰  (7)

wherein R¹³ may be the same or different and each independentlyrepresents a hydrogen atom or a linear, branched or cyclic C₁₋₂₀ alkylor aryl group, with the proviso that the hydrogen atom contained in thealkyl or aryl group may be substituted with a hydroxy or amino group, nstands for an integer from 0 to 3, and X¹⁰ represents a p-valentheterocyclic compound containing a nitrogen atom.

Examples of R¹² include hydrogen atom, and methyl, ethyl, propyl,isopropyl, butyl, isobutyl, pentyl, hexyl, heptyl, octyl, decyl,dodecyl, octadecyl, cyclohexyl, phenyl and tolyl groups.

Examples of the basic catalyst represented by the formula (6) includeammonia, methylamine, ethylamine, propylamine, butylamine, pentylamine,dodecylamine, octadecylamine, isopropylamine, t-butylamine,ethylenediamine, 1,2-diaminopropane, 1,3-diaminopropane,hexamethylenediamine, dimethylamine, diethylamine, dipropylamine,diisopropylamine, dibutylamine, trimethylamine, triethylamine,tripropylamine, tributylamine, N,N-dimethyloctylamine, triethanolamine,cyclohexylamine, aniline, N-methylaniline, diphenylamine and toluidines.

Examples of R¹³ include hydrogen atom and methyl, ethyl, propyl,isopropyl, butyl, isobutyl, pentyl, hexyl, heptyl, octyl, decyl,dodecyl, octadecyl, cyclohexyl, phenyl, tolyl, amino, methylamino,ethylamino, propylamino, butylamino, pentylamino, dodecylamino,octadecylamino, isopropylamino, t-butylamino, dimethylamino,diethylamino, dipropylamino, diisopropylamino, dibutylamino,N,N-dimethyloctylamino, cyclohexylamino and diphenylamino groups.

Examples of X¹⁰ include pyrrolidine, piperidine, morpholine, pyridine,pyridazine, pyrimidine, pyrazine and triazine.

Examples of the basic catalyst represented by the formula (7) includepyrrolidine, piperidine, morpholine, pyridine, picolines,phenylpyridines, N,N-dimethylaminopyridine, pyridazine, pyrimidine,pyrazine and triazine.

Of the above compounds, ammonia, methylamine, ethylamine, propylamine,isopropylamine, pyrrolidine, piperidine, morpholine and pyridine areespecially preferred as the basic catalyst. The basic catalyst may beused either singly or in combination.

The basic catalyst may be mixed with the silane compound or compoundsrepresented by the formula (4) and the structure-directing agentrepresented by the formula (5). The amount of the basic catalyst ispreferably from 0.01 to 20 mols, more preferably from 0.05 to 10 molsper mol of the silane compound or compounds represented by the formula(4).

When a zeolite sol is prepared by hydrolysis and condensation of thesilane compound(s) of the formula (4), water for hydrolysis is requiredas well as the silane compound(s), the structure-directing agent, andthe basic catalyst. Water may be added in an amount of from 0.1 to 100times the weight, more preferably from 0.5 to 20 times the weight, basedon the weight of the silane compound.

When a zeolite sol is prepared by hydrolysis and condensation of thesilane compound(s) of the formula (4), a solvent such as alcohol may beadded as well as water. Examples of the solvent include methanol,ethanol, isopropyl alcohol, butanol, propylene glycol monomethyl ether,propylene glycol monopropyl ether, propylene glycol monopropyl etheracetate, ethyl lactate and cyclohexanone. The solvent may be added in anamount of preferably from 0.1 to 100 times the weight, more preferablyform 0.5 to 20 times the weight, based on the weight of the silanecompound.

The hydrolysis reaction time is preferably from 1 to 100 hours, morepreferably from 10 to 70 hours, while the temperature is preferably from0 to 50° C., more preferably form 15 to 30° C. The heat treatment afterthe hydrolysis is performed at a temperature of preferably 30° C. orgreater, more preferably 50° C. or greater but not greater than 75° C.for preferably from 1 to 100 hours, more preferably from 10 to 70 hours.When the heat treatment temperature after hydrolysis is too low,transition from the aggregate of silicate ion to zeolite fine crystalsmay not occur easily and physical property-improving effect of theporous film forming composition may not be expected. When the heattreatment temperature exceeds 75° C., on the other hand, zeolitecrystals may grow to even a particle size of 50 nm or greater. Use ofsuch large crystals for the core may cause surface roughening of a filmthus formed or interfere with the formation of the shell.

The zeolite sol thus obtained may comprise fine particles having anaverage particle size of from 3 to 50 nm. It has markedly highmechanical strength because it has a similar crystal structure to thatof zeolite having a particle size of 50 nm or greater. Since theseparticles have a uniform and microporous crystal structure, they haveexcellent mechanical strength even though pores are distributed at aconsiderably high rate in the whole film thus formed.

(II) Core Containing Inorganic Silica or Organic Silica

On the other hand, inorganic or organic silica is also usable as thematerial for the core of the invention. It is industrially veryadvantageous material because it can be prepared easily in a short timecompared with zeolite. Organic silicon oxide fine particles containing,in the core thereof, inorganic silica or organic silica can have highmechanical strength.

As is apparent from the example of a bulk film prepared by CVD, thesilicon oxide material or particle has higher mechanical strength as thedensity of their Si—O—Si bond is higher. The organic silicon oxide fineparticles to be used for the core, can be preferably prepared using ahydrolyzable silane compound or compounds, containing a compoundrepresented by the following formula (4):

Si(OR¹⁰)₄  (4)

wherein R¹⁰ may be the same or different and each independentlyrepresents a linear or branched C₁₋₄ alkyl group.It is preferred because it can provide organic silicon oxide fineparticles having a high Si—O—Si density among conventional used ones.They may subsidiarily contain one or more compounds represented by thefollowing formula:

R¹⁴ _(r)Si(OR¹⁵)_(4-r)  (8)

wherein R¹⁴ may be the same or different and each independentlyrepresents a linear or branched C₁₋₆ alkyl group which may have asubstituent; R¹⁵, if there are a plurality of R¹⁵, may be the same ordifferent and each independently represents a linear or branched C₁₋₄alkyl group; and r stands for an integer from 1 to 3.

Incorporation of such a compound of the formula (8) may be effective forreducing a dielectric constant.

Specific examples of the silane compound represented by the formula (4)used preferably for the formation of the inorganic or organic silicainclude, but not limited to, tetramethoxysilane, tetraethoxysilane,tetrapropoxysilane, tetrabutoxysilane, tetraisopropoxysilane,tetraisobutoxysilane, triethoxymethoxysilane, tripropoxymethoxysilane,tributoxymethoxysilane, trimethoxyethoxysilane, trimethoxypropoxysilane,and trimethoxybutoxysilane. Examples of the silane compound representedby the formula (8) include methyltrimethoxysilane,methyltriethoxysilane, methyltri-n-propoxysilane,methyltri-i-propoxysilane, methyltri-n-butoxysilane,methyltri-s-butoxysilane, methyltri-i-butoxysilane,methyltri-t-butoxysilane, ethyltrimethoxysilane, ethyltriethoxysilane,ethyltri-n-propoxysilane, ethyltri-i-propoxysilane,ethyltri-n-butoxysilane, ethyltri-s-butoxysilane,ethyltri-i-butoxysilane, ethyltri-t-butoxysilane,n-propyltrimethoxysilane, n-propyltriethoxysilane,n-propyltri-n-propoxysilane, n-propyltri-i-propoxysilane,n-propyltri-n-butoxysilane, n-propyltri-s-butoxysilane,n-propyltri-i-butoxysilane, n-propyltri-t-butoxysilane,propyltrimethoxysilane, i-propyltriethoxysilane,propyltri-n-propoxysilane, i-propyltri-i-propoxysilane,propyltri-n-butoxysilane, i-propyltri-s-butoxysilane,propyltri-i-butoxysilane, i-propyltri-t-butoxysilane,n-butyltrimethoxysilane, n-butyltriethoxysilane,n-butyltri-n-propoxysilane, n-butyltri-i-propoxysilane,n-butyltri-n-butoxysilane, n-butyltri-s-butoxysilane,n-butyltri-i-butoxysilane, n-butyltri-t-butoxysilane,i-butyltrimethoxysilane, i-butyltriethoxysilane,i-butyltri-n-propoxysilane, i-butyltri-i-propoxysilane,i-butyltri-n-butoxysilane, i-butyltri-s-butoxysilane,i-butyltri-i-butoxysilane, i-butyltri-t-butoxysilane,s-butyltrimethoxysilane, s-butyltriethoxysilane,s-butyltri-n-propoxysilane, s-butyltri-i-propoxysilane,s-butyltri-n-butoxysilane, s-butyltri-s-butoxysilane,s-butyltri-i-butoxysilane, s-butyltri-t-butoxysilane,t-butyltrimethoxysilane, t-butyltriethoxysilane,t-butyltri-n-propoxysilane, t-butyltri-i-propoxysilane,t-butyltri-n-butoxysilane, t-butyltri-s-butoxysilane,t-butyltri-i-butoxysilane, t-butyltri-t-butoxysilane,dimethyldimethoxysilane, dimethyldiethoxysilane,dimethyldi-n-propoxylsilane, dimethyldi-i-propoxysilane,dimethyldi-n-butoxysilane, dimethyldi-s-butoxysilane,dimethyldi-i-butoxysilane, dimethyldi-t-butoxysilane,diethyldimethoxysilane, diethyldiethoxysilane,diethyldi-n-propoxylsilane, diethyldi-i-propoxysilane,diethyldi-n-butoxysilane, diethyldi-s-butoxysilane,diethyldi-i-butoxysilane, diethyldi-t-butoxysilane,di-n-propyldimethoxysilane, di-n-propyldiethoxysilane,di-n-propyldi-n-propoxylsilane, di-n-propyldi-i-propoxysilane,di-n-propyldi-n-butoxysilane, di-n-propyldi-s-butoxysilane,di-n-propyldi-i-butoxysilane, di-n-propyldi-t-butoxysilane,di-i-propyldimethoxysilane, di-i-propyldiethoxysilane,di-i-propyldi-n-propoxylsilane, di-i-propyldi-i-propoxysilane,di-i-propyldi-n-butoxysilane, di-i-propyldi-s-butoxysilane,di-i-propyldi-i-butoxysilane, di-i-propyldi-t-butoxysilane,di-n-butyldimethoxysilane, di-n-butyldiethoxysilane,di-n-butyldi-n-propoxylsilane, di-n-butyldi-i-propoxysilane,di-n-butyldi-n-butoxysilane, di-n-butyldi-s-butoxysilane,di-n-butyldi-i-butoxysilane, di-n-butyldi-t-butoxysilane,di-i-butyldimethoxysilane, di-i-butyldiethoxysilane,di-i-butyldi-n-propoxylsilane, di-i-butyldi-i-propoxysilane,di-i-butyldi-n-butoxysilane, di-i-butyldi-s-butoxysilane,di-i-butyldi-i-butoxysilane, di-i-butyldi-t-butoxysilane,di-s-butyldimethoxysilane, di-s-butyldiethoxysilane,di-s-butyldi-n-propoxylsilane, di-s-butyldi-i-propoxysilane,di-s-butyldi-n-butoxysilane, di-s-butyldi-s-butoxysilane,di-s-butyldi-i-butoxysilane, di-s-butyldi-t-butoxysilane,di-t-butyldimethoxysilane, di-t-butyldiethoxysilane,di-t-butyldi-n-propoxylsilane, di-t-butyldi-i-propoxysilane,di-t-butyldi-n-butoxysilane, di-t-butyldi-s-butoxysilane,di-t-butyldi-i-butoxysilane, di-t-butyldi-t-butoxysilane,trimethylmethoxysilane, trimethylethoxysilane,trimethyl-n-propoxysilane, trimethyl-i-propoxysilane,trimethyl-n-butoxysilane, trimethyl-s-butoxysilane,trimethyl-i-butoxysilane, trimethyl-t-butoxysilane,triethylmethoxysilane, triethylethoxysilane, triethyl-n-propoxylsilane,triethyl-i-propoxysilane, triethyl-n-butoxysilane,triethyl-s-butoxysilane, triethyl-i-butoxysilane,triethyl-t-butoxysilane, tri-n-propylmethoxysilane,tri-n-propylethoxysilane, tri-n-propyl-n-propoxysilane,tri-n-propyl-i-propoxysilane, tri-n-propyl-n-butoxysilane,tri-n-propyl-s-butoxysilane, tri-n-propyl-i-butoxysilane,tri-n-propyl-t-butoxysilane, tri-i-propylmethoxysilane,tri-i-propylethoxysilane, tri-i-propyl-n-propoxylsilane,tri-i-propyl-i-propoxysilane, tri-i-propyl-n-butoxysilane,tri-i-propyl-s-butoxysilane, tri-i-propyl-i-butoxysilane,tri-i-propyl-t-butoxysilane, tri-n-butylmethoxysilane,tri-n-butylethoxysilane, tri-n-butyl-n-propoxylsilane,tri-n-butyl-i-propoxysilane, tri-n-butyl-n-butoxysilane,tri-n-butyl-s-butoxysilane, tri-n-butyl-i-butoxysilane,tri-n-butyl-t-butoxysilane, tri-i-butylmethoxysilane,tri-i-butylethoxysilane, tri-i-butyl-n-propoxylsilane,tri-i-butyl-i-propoxysilane, tri-i-butyl-n-butoxysilane,tri-i-butyl-s-butoxysilane, tri-i-butyl-i-butoxysilane,tri-i-butyl-t-butoxysilane, tri-s-butylmethoxysilane,tri-s-butylethoxysilane, tri-s-butyl-n-propoxylsilane,tri-s-butyl-i-propoxysilane, tri-s-butyl-n-butoxysilane,tri-s-butyl-s-butoxysilane, tri-s-butyl-i-butoxysilane,tri-s-butyl-t-butoxysilane, tri-t-butylmethoxysilane,tri-t-butylethoxysilane, tri-t-butyl-n-propoxylsilane,tri-t-butyl-i-propoxysilane, tri-t-butyl-n-butoxysilane,tri-t-butyl-s-butoxysilane, tri-t-butyl-butoxysilane andtri-t-butyl-t-butoxysilane.

According to the method of the invention, one or more of the silanecompounds may be used as a mixture.

When a mixture of the compound(s) of the formula (4) and the compound(s)of the formula (8) is used as a raw material for the synthesis of thecore, the Si—O—Si density inside the core is preferably high in order toachieve sufficient strength. An amount of the compound(s) of the formula(4) is therefore preferably 50 mol % or greater of the total amount ofthe mixture of the compound(s) of the formula (4) and the compound(s) ofthe formula (8).

Organic silicon oxide fine particles having the above core can beobtained by hydrolysis and condensation of the above hydrolyzable silanein the presence of an acid or basic catalyst. In order to increase theSi—O—Si bond density (condensation degree) to achieve high mechanicalstrength, the basic catalyst may be preferred.

Many compounds such as alkali metal hydroxide, organic ammoniumhydroxide and amine are known as the basic catalyst. The basic catalystmay be used singly or in combination. Specific examples of the preferredbasic catalyst include alkali metal hydroxides such as lithiumhydroxide, sodium hydroxide, potassium hydroxide, and cesium hydroxide;ammonium salts such as tetramethylammonium hydroxide, choline,tetraethylammonium hydroxide, tetrapropylammonium hydroxide,tetrabutylammonium hydroxide, tetrapentylammonium hydroxide, andtetrahexylammonium hydroxide; and amines such as DBU, DABCO,triethylamine, diethylamine, pyridine, piperidine, piperazine andmorpholine.

The basic catalyst is used in an amount of preferably from 1 to 50 mol%, more preferably from 5 to 30 mol %, still more preferably from 10 to20 mol % based on the total amount of the hydrolyzable silane. Anexcessively large amount of the catalyst may make it difficult to obtaina low k film because growth of organic silicon oxide fine particles maybe inhibited and sufficient growth may not be expected. An excessivelysmall amount, on the other hand, may make it impossible to achieveintended strength because of insufficient condensation of siloxane.

Fine particles having higher mechanical strength can be obtained, forexample, by using, as described below, a hydrophobic quaternary ammoniumhydroxide and a hydrophilic quaternary ammonium hydroxide in combinationas the catalyst. The hydrophilic catalyst is an alkali metal hydroxideor a quaternary ammonium hydroxide represented by the following formula(9):

(R¹⁶)₄N⁺OH⁻  (9)

wherein R¹⁶ may be the same or different and each independentlyrepresents a C₁₋₂ hydrocarbon group which may contain an oxygen atom;and the cationic moiety [(R¹⁶)₄N⁺]satisfies the following equation (A):

(N+O)/(N+O+C)≧1/5  (A)

wherein N, O, and C represent the number of nitrogen, oxygen and carbonatoms contained in the cationic moiety, respectively. The hydrophobiccatalyst is preferably a compound represented by the following formula(10):

(R¹⁷)₄N⁺OH⁻  (10)

wherein R¹⁷ may be the same or different and each independentlyrepresents a linear or branched C₁₋₈ alkyl group with the proviso thatall R¹⁷ do not represent a methyl group at the same time; and thecationic moiety [(R¹⁷⁾ ₄N⁺] satisfies the following equation (B):

(N+O)/(N+O+C)<1/5  (B)

wherein N, O, and C represent the number of nitrogen, oxygen and carbonatoms contained in the cationic moiety, respectively.

The organic silicon oxide fine particles prepared in such a manner mayshow higher strength compared with those prepared in the conventionalmanner.

When condensation is performed using the hydrophobic basic catalyst andthe hydrophilic basic catalyst in combination, the hydrophilic basiccatalyst is added preferably in an amount of from 0.2 to 2.0 mols permol of the hydrophobic basic catalyst.

The hydrolysis and condensation reaction of the hydrolyzable silanesrequires addition of water for hydrolysis and an amount of water to beadded to the reaction system is preferably from 0.5 to 100 times themole, more preferably from 1 to 10 times the mole necessary forhydrolyzing the silane compounds completely.

When the hydrolyzable silane is subjected to hydrolysis and condensationto obtain a polymer solution, the reaction system may contain, inaddition to water, a solvent such as an alcohol corresponding to thealkoxy group of the silane compound. Examples include methanol, ethanol,isopropyl alcohol, butanol, propylene glycol monomethyl ether, propyleneglycol monopropyl ether, propylene glycol monopropyl ether acetate,ethyl lactate and cyclohexanone.

The solvent other than water is added in an amount of preferably from0.1 to 500 times the weight, more preferably from 1 to 100 times theweight, based on the weight of the silane compound.

Although the hydrolysis and condensation reaction of the silane compoundmay be performed under the conditions employed for the conventionalhydrolysis and condensation reaction, the reaction temperature may beset to fall within a range of usually from 0° C. to the boiling point ofan alcohol generated by the hydrolysis and condensation, preferably fromroom temperature (15° C.) to 80° C.

In a more convenient reaction method, silica fine particles may form andgrow when the hydrolyzable silane substance(s) or solution dissolved inthe above solvent is added to an aqueous solution (in some cases, mixedwith an organic solvent) of the basic catalyst adjusted to the abovereaction temperature. The addition may be usually dropwise orintermittent is usually for from 10 minutes to 24 hours, more preferablyfrom 30 minutes to about 8 hours.

Then, a formation reaction of the shell portion, which will be describedin detail later, can be conducted successively. Formation of the shellon the periphery of the core comprising the inorganic or organic silicamay be started after a so-called aging reaction, that is, maintenance ofconditions under which the hydrolysis and condensation reaction proceedsfor preferably from 5 minutes to 4 hours, more preferably from 10minutes to 1 hour after completion of the addition of the hydrolyzablesilane for the formation of the core portion. It is also possible tochange the composition continuously by carrying out the reaction whilegradually changing the composition of the raw material from that forforming the core to that for forming the shell, or carrying out thereaction while partially overlapping the raw material for the core withthe raw material for the shell.

[Shell]

Next, a shell is formed so as to completely cover the periphery of theinorganic or organic silicon oxide fine particles obtained by the aboveprocess as the core.

The shell has a ratio T/Q greater than that of the core wherein T is thenumber of silicon atoms having at least one bond directly attached to acarbon atom and Q is the number of silicon atoms having all of the fourbonds attached to an oxygen atom, for the purpose of reducing chemicalreactivity of silicon atoms constituting the core, thereby makingchemical stability of the shell greater than that of the core. Inaddition, the shell preferably consists essentially of silicon atomseach having at least one bond to which a carbon atom is directlyattached to prevent occurrence of a partially weak portion, therebyimparting high stability to the shell. This means that the T/Q ratio ispreferably 95/5 or greater, more preferably 98/2 or greater. Since theshell should be a dense film covering the core completely, it contains askeleton derived from a multinuclear hydrolyzable silane which containshydrolyzable-group-having silicon atoms bound via a hydrocarbon groupwhich will be described later.

As another expected effect of the shell, it is used for impartingdeformability to the surface of the particles in order to widen acontact area between particles to heighten the interparticle bindings atthe time of film formation. The skeleton derived from a multinuclearhydrolyzable silane having a silicon atom directly attached to ahydrocarbon group is expected to have a function of increasing thecontact surface area between the particles at the time of filmformation.

As described above, after completion of the formation of the core and ifnecessary, after the aging step, it is preferred to carry out the shellformation successively. When the core is isolated or it is left to standfor a long period of time, aggregation of fine particles may possiblyoccur. The aging may be performed by maintaining the hydrolysis andcondensation reaction conditions of the core for preferably from 5minutes to 4 hours, more preferably from 10 minutes to 1 hour aftercompletion of the addition of the hydrolyzable silane as the material ofthe core. The aging may be effective for forming a shell with a thinnerlayer and reflecting the mechanical strength of the core in theresulting film. The shell is preferably formed using a basic catalyst toserve as a protective film having high density. A shell with highdensity can be obtained by starting the formation of the shell on fineparticles of the core, which have been just prepared and therefore have,on the surface thereof, very active silanol groups, immediately afterpreparation or after re-adjustment of the reaction conditions, therebycausing an efficient reaction between the shell-forming material and thesurface of the fine particles.

Formation of a shell by using the catalyst adsorbed to the surface ofthe fine particles during core formation is effective for suppressingthe generation of new fine particles composed only of the shell-formingmaterial.

A shell can be formed on the surface of zeolite by adding dropwise asolution containing the raw material of the shell portion to the zeolitefine particle solution of the core successively after preparationthereof by the above zeolite preparation process. During the formation,an alcohol solvent may be added as needed or a basic catalyst havinghigh hydrophilicity may be added further. When gelation occurs duringthe shell-forming operation, addition of alcohol can prevent gelationeffectively. The basic catalyst having high hydrophilicity may beeffective for forming a shell having a high crosslink density and highchemical stability.

When the silica obtained using the acid catalyst is used as the core,the catalyst system should be changed from an acid to a base forobtaining a shell having a high density and therefore having highchemical stability.

A shell can be formed on or above the silica core produced in thepresence of the basic catalyst, using an alkoxysilane as a raw materialwithout substantial re-adjustment of the reaction mixture such asaddition of a new catalyst. In particular, a catalyst design forobtaining a core having high mechanical strength and a catalyst designfor obtaining a shell having a high crosslink density and thereforeproviding high chemical stability are the same so that it is preferredto successively add dropwise the shell-forming material to the reactionsystem used for the formation of the core.

Compared with the core component, the fundamental structure of the shellcomponent has a low polarity and has accordingly a property of having alow dielectric constant. It has low mechanical strength and is likely tocollapse so that it is not suited for forming pores mainly by making useof interparticle spaces. As a result, the film produced by using it hasa high dielectric constant or even if it has a low dielectric constant,it tends to have very low mechanical strength. Even if the combinationof the core component and the shell component is the same, balance as awhole film between dielectric constant and strength differs, dependingon the size of fine particles or thickness of the shell. The combinationproviding an optimum balance should be adopted as needed depending onthe application purpose.

When a shell is formed on the same core, the shell is preferably not sothick in order to achieve a low dielectric constant. For this purpose,it is preferred to carry out, after completion of the addition of acore-forming material in a core formation step, the aging step and thenstart the addition of a shell-forming material.

Use of a shell having a certain thickness, on the other hand, causes aslight increase in dielectric constant, but can increase the filmstrength after baking because a contact area between particles widensdue to deformability of the shell. When formation of a shell having acertain thickness is desired, dropwise addition of a shell-formingmaterial may be started prior to the completion of the dropwise additionof a core-forming material to form an intermediate layer having agradient composition. Alternatively, an intermediate-layer-formingmaterial may be added dropwise separately after completion of thedropwise addition of a core-forming material to form an intermediatelayer and then, a shell may be formed as the outer layer of theresulting intermediate layer.

The thickness of the intermediate layer is preferably from 0 to 0.5 nm,more preferably from 0 to 0.1 nm. Formation of the intermediate layer iseffective for imparting chemical stability to the resulting film withoutsignificantly deteriorating the mechanical strength of it.

The material used for the formation of the shell of the invention is ahydrolyzable silane compound or compounds, containing a hydrolyzablesilane having two or more silicon atoms substituted with a hydrolyzablegroup and linked via a carbon chain or a chain containing a silicon atombetween some carbons.

Examples of the hydrolyzable silane compound or compounds, having two ormore silicon atoms substituted with a hydrolyzable group and linked viaa carbon chain or a chain containing a silicon atom between some carbonsand used preferably for the formation of the shell include one or morehydrolyzable compounds represented the following formula (1) or (8):

{R¹ _(n)X¹ _(3-n)Si—[(Y²)—(SiR² _(m)X² _(2-m))]_(p)}_(q)—(Y³)—SiR³_(t)X³ _(3-t)  (1)

wherein X¹ to X³ each independently represents a hydrolyzable groupselected from the group consisting of a hydrogen atom, halogen atoms andC₁₋₄ alkoxy groups; R¹ to R³ each independently represents a C₁₋₂₀ alkylgroup or a C₆₋₁₀ aryl group; Y² and Y³ each independently represents asubstituted or unsubstituted C₁₋₆ hydrocarbon group having q+1valencies, a C₅₋₂₀ cycloalkane group which has q+1 valencies and maycontain a fused ring structure, or a C₆₋₂₀ aromatic group having q+1valencies; m each independently represents an integer from 0 to 2; neach independently represents an integer from 0 to 2; p eachindependently represents an integer from 0 to 4; q each independentlyrepresents an integer of 1 or greater; and t each independentlyrepresents an integer from 0 to 2;

R¹⁴ _(r)Si(OR¹⁵)_(4-r)  (8)

wherein R¹⁴ may be the same or different and each independentlyrepresents a linear, branched or cyclic C₁₋₆ alkyl group which may havea substituent; R¹⁵, when there are a plurality of R¹⁵, may be the sameor different and each independently represents a linear or branched C₁₋₄alkyl group; and r stands for an integer from 1 to 3. With regard to Y²and Y³ in the formula (1), examples of the C₁₋₆ hydrocarbon group havingvalencies of q+1 include methylene, ethylene, propylene, butylene andhexylene; those of the C₅₋₂₀ cycloalkane group having valencies of q+1include groups having a cyclopentane ring structure and groups having acyclohexane ring structure; those of the cycloalkane group containing afused ring structure and valencies of q+1 include groups having anorbornane ring structure, groups having a bicyclodecane ring structure,and groups having an adamantane ring structure; those of the C₆₋₂₀aromatic group having valencies of q+1 include groups having a benzenering structure and groups having an anthracene ring structure. Examplesof the substituent of Y² or Y³ include methyl, ethyl, propyl, and butylgroups. In the formula (1), q may stand for from 0 to 20, preferablyfrom 0 to 3. Examples of the substituent which R¹⁴ may have in theformula (8) include methyl, ethyl, n-propyl, i-propyl, n-butyl, s-butyl,i-butyl, and t-butyl groups.

The hydrolyzable silane compound(s) as represented by the formula (1)and having two or more silicon atoms substituted with a hydrolyzablegroup and linked via a carbon chain or a chain containing a silicon atombetween some carbons can prevent an increase in the number ofsubstituents attached to silicon which do not participate incrosslinking. Accordingly, addition of the hydrolyzable silanecompound(s) is effective for densifying a layer of the shell and theresulting shell is useful for enhancing chemical resistance. When thehydrolyzable silane compound(s) having two or more silicon atomssubstituted with a hydrolyzable group and linked via a carbon chain or achain containing a silicon atom between some carbons is mixed with acompound other than a multinuclear hydrolyzable silane, a ratio of themultinuclear hydrolyzable silane compound in all the hydrolyzable silanecompounds is preferably 25% or greater, more preferably 40% or greater,still more preferably 50% or greater, each in terms of a silicon atom(the number of silicon atoms).

Of the compounds represented by the formula (1), more preferred arecompounds represented by the formulas (2) and (3):

wherein X⁴ to X⁹ each independently represents a hydrolyzable groupselected from the group consisting of hydrogen atom, halogen atoms andC₁₋₄ alkoxy groups; R⁴ to R⁹ each independently represents a C₁₋₂₀ alkylgroup or a C₆₋₁₀ aryl group; m each independently represents an integerfrom 0 to 2; n each independently represents an integer from 0 to 2; peach independently represents an integer from 0 to 4; r eachindependently represents an integer from 0 to 4; each independentlyrepresents an integer from 0 to 4; t each independently represents aninteger from 0 to 2; and u each independently represents an integer from0 to 4.

The skeletons represented by the formula (11) are shown below asspecific examples of the skeletons of the compounds represented by theformulas (2) and (3).

Specific examples of the hydrolyzable silane having the above skeletoninclude chain siloxanes such as1,3-dimethyl-1,1,3,3-tetramethoxydisiloxane,1,1,3-trimethyl-1,3,3-trimethoxydisiloxane,1,1,3,3-tetramethyl-1,3-dimethoxydisiloxane,1,3-dimethyl-1,1,3,3-tetraethoxydisiloxane,1,1,3-trimethyl-1,3,3-triethoxydisiloxane,1,1,3,3-tetramethyl-1,3-diethoxydisiloxane,1,3-dimethyl-1,1,3,3-tetrapropoxydisiloxane,1,1,3-trimethyl-1,3,3-tripropoxydisiloxane,1,1,3,3-tetramethyl-1,3-dipropoxydisiloxane,1,3-dimethyl-1,1,3,3-tetrabutoxydisiloxane,1,1,3-trimethyl-1,3,3-tributoxydisiloxane,1,1,3,3-tetramethyl-1,3-dibutoxydisiloxane,1,3,5-trimethyl-1,1,3,5,5-pentamethoxytrisiloxane,1,1,3,5-tetramethyl-1,3,5,5-tetramethoxytrisiloxane,1,1,3,5,5-pentamethyl-1,3,5-trimethoxytrisiloxane,1,3,5-trimethyl-1,1,3,5,5-pentaethoxytrisiloxane,1,1,3,5-tetramethyl-1,3,5,5-tetraethoxytrisiloxane,1,1,3,5,5-pentamethyl-1,3,5-triethoxytrisiloxane,1,3,5,7-tetramethyl-1,1,3,5,7,7-hexamethoxytetrasiloxane,1,1,3,5,7,7-hexamethyl-1,3,5,7-tetramethoxytetrasiloxane,1,3,5,7-teteramethyl-1,1,3,5,7,7-hexaethoxytetrasiloxane, and1,1,3,5,7,7-hexamethyl-1,3,5,7-tetraethoxytetrasiloxane and in addition,include bis(trimethoxysilyl)methane, bis(triethoxysilyl)methane,bis(methyldimethoxysilyl)methane, bis(methyldiethoxysilyl)methane,bis(dimethylmethoxysilyl)methane, bis(dimethylethoxysilyl)methane,1,2-bis(trimethoxysilyl)ethane, 1,2-bis(triethoxysilyl)ethane,1,2-bis(methyldimethoxysilyl)ethane, 1,2-bis(methyldiethoxysilyl)ethane,1,2-bis(dimethylmethoxysilyl)ethane, 1,2-bis(dimethylethoxysilyl)ethane,1,3-bis(trimethoxysilyl)propane, 1,3-bis(triethoxysilyl)propane,1,3-bis(methyldimethoxysilyl)propane,1,3-bis(methyldiethoxysilyl)propane,1,3-bis(dimethylmethoxysilyl)propane,1,3-bis(dimethylethoxysilyl)propane, 1,4-bis(trimethoxysilyl)butane,1,4-bis(triethoxysilyl)butane, 1,4-bis(methyldimethoxysilyl)butane,1,4-bis(methyldiethoxysilyl)butane, 1,4-bis(dimethylmethoxysilyl)butane,1,4-bis(dimethylethoxysilyl)butane, 1,5-bis(trimethoxysilyl)pentane,1,5-bis(triethoxysilyl)pentane, 1,5-bis(methyldimethoxysilyl)pentane,1,5-bis(methyldiethoxysilyl)pentane,1,5-bis(dimethylmethoxysilyl)pentane,1,5-bis(dimethylethoxysilyl)hexane, 1,6-bis(trimethoxysilyl)hexane,1,6-bis(triethoxysilyl)hexane, 1,6-bis(methyldimethoxysilyl)hexane,1,6-bis(methyldiethoxysilyl)hexane, 1,6-bis(dimethylmethoxysilyl)hexane,1,6-bis(dimethylethoxysilyl)hexane, 1,2-bis(trimethoxysilyl)benzene,1,2-bis(triethoxysilyl)ethane, 1,2-bis(methyldimethoxysilyl)benzene,1,2-bis(methyldiethoxysilyl)benzene,1,2-bis(dimethylmethoxysilyl)benzene,1,2-bis(dimethylethoxysilyl)benzene, 1,3-bis(triimethoxysilyl)benzene,1,3-bis(triethoxysilyl)ethane, 1,3-bis(methyldimethoxysilyl)benzene,1,3-bis(methyldiethoxysilyl)benzene,1,3-bis(dimethylmethoxysilyl)benzene,1,3-bis(dimethylethoxysilyl)benzene, 1,4-bis(trimethoxysilyl)benzene,1,4-bis(triethoxysilyl)ethane, 1,4-bis(methyldimethoxysilyl)benzene,1,4-bis(methyldiethoxysilyl)benzene,1,4-bis(dimethylmethoxysilyl)benzene, and1,4-bis(dimethylethoxysilyl)benzene.

These compounds have crosslinking groups at both ends thereof and aflexible structure at an intermediate portion thereof so that they canbe easily structured and therefore have an improved film formationproperty compared with a simple silane compound. In particular, whencomponents at the intermediate portion are bonded via an alkylene chainor phenylene chain, such a compound can form a shell having highhydrophobicity compared with a hydrolysis condensate of a compoundhaving a siloxane bond or a silane compound.

The following are skeleton examples of the multinuclear hydrolyzablesilane compound represented by the following formula (12) which can beused preferably in addition to the above ones.

Specific examples of the hydrolyzable silane compound having two or moresilicon atoms substituted with a hydrolyzable group and linked via acarbon chain or a chain containing a silicon atom between some carbonsand having the above cyclic structure include1,3,5-trimethyl-1,3,5-trimethoxy-1,3,5-trisilacyclohexane,1,3,5-trimethyl-1,3,5-triethoxy-1,3,5-trisilacyclohexane,1,3,5-trimethyl-1,3,5-tripropoxy-1,3,5-trisilacyclohexane,1,3,5-trimethyl-1,3,5-tributoxy-1,3,5-trisilacyclohexane,1,3,5,7-tetramethyl-1,3,5,7-tetramethoxy-1,3,5,7-tetrasilacyclooctane,1,3,5,7-tetramethyl-1,3,5,7-tetraethoxy-1,3,5,7-tetrasilacyclooctane,1,3,5,7-tetramethyl-1,3,5,7-tetrapropoxy-1,3,5,7-tetrasilacyclooctane,1,3,5,7-tetramethyl-1,3,5,7-tetrabutoxy-1,3,5,7-tetrasilacyclooctane,1,3,5,7-tetramethyl-1,3,5,7-tetramethoxy-1,3,5,7-tetrasila-2,6-dioxacyclooctane,1,3,5,7-tetramethyl-1,3,5,7-tetraethoxy-1,3,5,7-tetrasila-2,6-dioxacyclooctane,1,3,5,7-tetramethyl-1,3,5,7-tetrapropoxy-1,3,5,7-tetrasila-2,6-dioxacyclooctane,1,3,5,7-tetramethyl-1,3,5,7-tetrabutoxy-1,3,5,7-tetrasila-2,6-dioxacyclooctane,1,3,6,9-tetramethyl-1,3,6,9-teteramethoxy-1,3,6,9-tetrasila-2,8-dioxacyclodecane,1,3,6,9-tetramethyl-1,3,6,9-tetraethoxy-1,3,6,9-tetrasila-2,8-dioxacyclodecane,1,3,6,9-tetramethyl-1,3,6,9-tetrapropoxy-1,3,6,9-tetrasila-2,8-dioxacyclodecane,and1,3,6,9-tetramethyl-1,3,6,9-tetrabutoxy-1,3,6,9-tetrasila-2,8-dioxacyclodecane.

As the preferable hydrolyzable silane compound, which has two or moresilicon atoms substituted with a hydrolyzable group and linked via acarbon chain or a chain having one silicon atom between some carbonatoms, other than the above compounds, multi-branched multinuclearhydrolyzable silane compounds can be mentioned. Specific skeletonexamples of them are represented by the following formula (13):

Some of the hydrolyzable silanes exemplified above contain an aromaticring. Introduction of an aromatic ring is effective for improving thecarbon concentration without deteriorating the heat resistance. Inaddition, an aromatic radical is, similar to a silyl radical, stable andSi and an aromatic ring tend to form a bond so that such a hydrolyzablesilane is effective for improving strength.

The hydrolyzable silane represented by the formula (8) is a preferredcompound here, including those exemplified above as a compound which canbe added subsidiarily upon formation of the core.

When the hydrolyzable silane to be used for formation of the shell isdesigned in such a manner that it essentially contains a hydrolyzablesilane compound having two or more silicon atoms substituted with ahydrolyzable group and linked via a carbon chain or a chain containingone silicon atom between some carbon atoms and at the same time, a ratio(T/Q) of the number (T) of the silicon atoms having at least one bonddirectly attached to a carbon atom to the number (Q) of silicon atomshaving all of the four bonds attached to an oxygen atom is greater thanthat in the core, chemical stability can be achieved due to thehydrophobicity of the invention imparted to the shell. Presence ofportions having low stability is not preferred for achieving higherstability. When a mixture of hydrolyzable silane compounds is used forthe formation of a shell, the hydrolyzable silane contained in themixture may preferably consist essentially of a hydrolyzable silanecompound or compounds substituted with a substituent having a carbonatoms directly attached to a silicon atom. The term “consist essentiallyof” as used herein may include that 95 mol % or greater, in terms ofsilicon (the number of silicon atoms), more preferably 98 mol % orgreater, still more preferably 100% of the hydrolyzable silanecompound(s) contained in the mixture is a hydrolyzable silanesubstituted with a substituent having a carbon atom directly attached toa silicon atom. This makes it possible to ensure a certain level ofchemical stability of the entire shell and prevent formation of aportion having weak chemical stability. As a result, the fine particlesin their entirety can have high chemical stability.

When the shell is formed by the dropwise addition of the hydrolyzablesilane compound, so-called aging time for a particularly long period oftime is not necessary after the dropwise addition, because the silanecompound reacts promptly after the dropwise addition. Long aging timehowever does not cause any marked deterioration. The film obtained bycarrying out neutralization termination after aging for more than 4hours after completion of the dropwise addition tends to have a reducedstrength. The film obtained by carrying out neutralization terminationwithin one hour tends to have high strength.

The minimum necessary amount of the hydrolyzable silane used for theshell layer can be determined by designing the thickness of the shelllayer to be 0.025 nm or greater on average in order to completely coverthe core with the shell layer. Under conditions for preparing silicafine particles having a particle size of 2 nm, particles are preparedwhile changing the weight ratio of (the core-forming material)/(theshell-forming material). As a result, formation of particles dependingon the chemical properties of the shell may be recognized at acore/shell weight ratio falling within a range of 90/10 or less. Theminimum necessary thickness of the shell layer assuming that the coreand the shell have the same density may be estimated at 0.025 nm. Whenthe amounts of hydrolyzable silane compounds used for the core and shellare compared in terms of silicon atoms (number of silicon atoms), theamount of the hydrolyzable silane compound(s) used for the shell is notgreater than the molar equivalent used for the core. This means that thenumber of silicon atoms contained in the core is preferably greater thanthat contained in the shell. When the molar equivalent of the silanecompound used for the shell exceeds that of the silane compound used forthe core, there is a danger of the high mechanical strength of the corenot being reflected sufficiently in the physical property of the entiresilica fine particles. A preferable amount of the hydrolyzable silaneused for the shell varies depending on the intended size of the fineparticles. The weight ratio (core/shell) of the hydrolyzable silanecompound for the core and that for the shell is preferably from 95/5 to50/50. When the fine particles have an average particle size of about 2nm, the weight ratio is preferably from 90/10 to 70/30.

When the hydrolysis and condensation reaction of the silane compound(s)for the formation of the shell is completed, a step of protecting asurface active silanol is preferably introduced. Described specifically,after neutralization reaction of the basic catalyst and prior todisappearance of crosslinking activity, more preferably immediatelyafter the neutralization reaction, a divalent or higher valentcarboxylic acid compound is added to protect the active silanol, or theneutralization reaction itself is performed with a divalent or highervalent carboxylic acid to simultaneously carry out neutralization andsilanol protection. Thus, the crosslinking activity can be frozen untilthe decomposition of the carboxylic acid at the time of film formation.

Examples of the preferable carboxylic acid having, in the moleculethereof, at least two carboxyl groups include oxalic acid, malonic acid,malonic anhydride, maleic acid, maleic anhydride, fumaric acid, glutaricacid, glutaric anhydride, citraconic acid, citraconic anhydride,itaconic acid, itaconic anhydride and adipic acid. The carboxylic acidacts effectively when added in an amount of preferably from 0.05 to 10mol %, more preferably from 0.5 to 5 mol %, each based on silicon unit.

[Film-Forming Composition]

The film-forming composition of the invention contains the organicsilicon oxide fine particles of the invention and an organic solvent.The film-forming composition can be prepared in accordance with theconventional preparation process (for example, JP 2005-216895A or JP2004-161535A) of a film-forming composition containing organic siliconoxide fine particles.

When the film-forming composition is used as a semiconductor insulatingfilm material which will be described later and an alkali metalhydroxide is used as the hydrophilic basic catalyst, demetallizationtreatment is inevitably performed in any stage of from the abovereaction termination to the preparation of a coating compositionsolution. Although there are many examples of the demetallizationtreatment, a method using an ion exchange resin or washing with anorganic solvent solution is usually employed. Such demetallizationtreatment is not essential when a silica sol is prepared using acombination of only ammonium catalysts not containing a metal impurity,but it is the common practice to add a demetallization treatment stepsimilarly.

In addition, a solvent such as water used for preparing a solutioncontaining the organic silicon oxide fine particles is usually replacedby a solvent for coating which will be described later. There are manyknown examples of this method. Even in the case where the organicsilicon oxide fine particles of the invention have been subjected to theabove stabilization treatment, it may not be preferred to remove thesolvent completely to isolate these particles.

Many solvents known as a solvent to be used for preparing a solution ofa film-forming coating composition are usable for the film-formingcomposition of the invention. Specific examples include aliphatichydrocarbon solvents such as n-pentane, isopentane, n-hexane, isohexane,n-heptane, 2,2,2-trimethylpentane, n-octane, isooctane, cyclohexane, andmethylcyclohexane; aromatic hydrocarbon solvents such as benzene,toluene, xylene, ethylbenzene, trimethylbenzene, methylethylbenzene,n-propylbenzene, isopropylbenzene, diethylbenzene, isobutylbenzene,triethylbenzene, diisopropylbenzene, and n-amylnaphthalene; ketonesolvents such as acetone, methyl ethyl ketone, methyl n-propyl ketone,methyl n-butyl ketone, methyl isobutyl ketone, cyclohexanone,2-hexanone, methylcyclohexanone, 2,4-pentanedione, acetonylacetone,diacetone alcohol, acetophenone, and fenthion; ether solvents such asethyl ether, isopropyl ether, n-butyl ether, n-hexyl ether, 2-ethylhexylether, dioxolane, 4-methyldioxolane, dioxane, dimethyldioxane, ethyleneglycol mono-n-butyl ether, ethylene glycol mono-n-hexyl ether, ethyleneglycol monophenyl ether, ethylene glycol mono-2-ethylbutyl ether,ethylene glycol dibutyl ether, diethylene glycol monomethyl ether,diethylene glycol dimethyl ether, diethylene glycol monoethyl ether,diethylene glycol diethyl ether, diethylene glycol monopropyl ether,diethylene glycol dipropyl ether, diethylene glycol monobutyl ether,diethylene glycol dibutyl ether, tetrahydrofuran,2-methyltetrahydrofuran, propylene glycol monomethyl ether, propyleneglycol dimethyl ether, propylene glycol monoethyl ether, propyleneglycol diethyl ether; propylene glycol monopropyl ether, propyleneglycol dipropyl ether, propylene glycol monobutyl ether, dipropyleneglycol dimethyl ether, dipropylene glycol diethyl ether, dipropyleneglycol dipropyl ether, and dipropylene glycol dibutyl ether, estersolvents such as diethyl carbonate, ethyl acetate, gamma-butyrolactone,gamma-valerolactone, n-propyl acetate, isopropyl acetate, n-butylacetate, isobutyl acetate, sec-butyl acetate, n-pentyl acetate,3-methoxybutyl acetate, methylpentyl acetate, 2-ethylbutyl acetate,2-ethylhexyl acetate, benzyl acetate, cyclohexyl acetate,methylcyclohexyl acetate, n-nonyl acetate, methyl acetoacetate, ethylacetoacetate, ethylene glycol monomethyl ether acetate, ethylene glycolmonoethyl ether acetate, diethylene glycol monomethyl ether acetate,diethylene glycol monoethyl ether acetate, diethylene glycolmono-n-butyl ether acetate, propylene glycol monomethyl ether acetate,propylene glycol monoethyl ether acetate, dipropylene glycol monomethylether acetate, dipropylene glycol monoethyl ether acetate, dipropyleneglycol mono-n-butyl ether acetate, glycol diacetate, methoxytriglycolacetate, ethyl propionate, n-butyl propionate, isoamyl propionate,diethyl oxalate, di-n-butyl oxalate, methyl lactate, ethyl lactate,n-butyl lactate, n-amyl lactate, diethyl malonate, dimethyl phthalate,and diethyl phthalate; nitrogen-containing solvents such asN-methylformamide, N,N-dimethylformamide, acetamide, N-methylacetamide,N,N-dimethylacetamide, N-methylpropionamide, and N-methylpyrrolidone,and sulfur-containing solvents such as dimethyl sulfide, diethylsulfide, thiophene, tetrahydrothiophene, dimethyl sulfoxide, sulfolaneand 1,3-propanesultone. The solvent may be used singly or incombination.

In some cases, a coating solution can be prepared by mixing a compoundhaving an external-forming property such as polyether or long-chainalkyltrimethylammonium salt (SDA: structure-directing agent) or aheat-decomposable compound for simply forming pores. As theheat-decomposable compound, sugars, poly(meth)acrylates, and hydrocarboncompounds having a boiling point of from 250 to 400° C. are preferred.

Dilution is finally performed to prepare a composition for obtaining anintended film. The degree of dilution differs depending on theviscosity, intended film thickness or the like. Dilution is usuallyperformed so that the amount of the solvent in the film composition maybe preferably from 50 to 99% by weight, more preferably from 75 to 98%by weight. The concentration of the organic silicon oxide fine particlesin the film-forming composition is preferably from 1 to 80% by weight,more preferably from 2 to 25% by weight.

As a material to be added to the film-forming composition, manyfilm-forming auxiliary components including a surfactant are known andany of them can fundamentally be used for the film-forming compositionof the invention. For example, a surfactant may be comprised by thefilm-forming composition preferably in an amount of from 0 to 3% byweight.

The film-forming composition of the present invention may contain, asthe polymer component of silicon, a polysiloxane prepared by anotherprocess. In order to achieve the advantage of the invention, the ratioof the polysiloxane prepared by another process is preferably 50% byweight or less, more preferably 20% by weight or less based on theweight of the organic silicon oxide fine particles.

[Porous Film]

A film of any film thickness can be formed by applying theporous-film-forming composition prepared in the above manner to asubstrate by spin-coating at an adequate rotation number. Thecomposition can be applied by not only spin-coating but also anothermethod such as scan-coating.

The actual film thickness is usually from about 0.1 to 1.0 μm, but thethickness is not limited thereto. A film having a greater thickness canalso be formed by application in a plurality of times.

The film thus formed can be made porous by a known manner. For example,a porous film can be obtained by removing the solvent by heating thefilm in an oven in a drying step (usually a step called “prebake” in asemiconductor process), preferably heating the film to from 50 to 150°C. for several minutes and then baking at from 350 to 450° C. for from 2to 60 minutes. The heating step (baking step) may be followed orreplaced by a step such as curing step to expose to an electron beam orlight. As the light, for example, an ultraviolet ray may be employed.

[Semiconductor Device]

The porous film obtained in such a manner can be used as an insulatingfilm in a semiconductor device in a known manner. The insulating film ismounted on a semiconductor device in a known manner. A semiconductordevice equipped with such a porous insulating film having both highmechanical strength and high chemical stability can exhibits highperformance and high reliability

EXAMPLES Synthesis Example 1

A mixture of 8.26 g of a 25% aqueous solution of tetramethylammoniumhydroxide, 34.97 g of ultrapure water, and 376.80 g of ethanol washeated to 60° C. in advance. A mixture of 19.48 g of tetramethoxysilaneand 17.44 g of methyltrimethoxysilane was added dropwise over 1 hour,followed by the dropwise addition of a mixture of 4.33 g of1,2-bis(trimethoxysilyl)ethane and 4.36 g of methyltrimethoxysilane tothe reaction mixture over 15 minutes. After completion of the dropwiseaddition, the reaction mixture was cooled to 40° C. or less andneutralized with an aqueous solution of maleic acid. After addition of150 g of propylene glycol propyl ether, the resulting mixture wasconcentrated at a temperature not greater than 40° C. under reducedpressure to distill off ethanol. Ethyl acetate (300 ml) was added,followed by washing three times with 200 ml of ultrapure water.Propylene glycol propyl ether (200 ml) was added and the resultingmixture was re-concentrated at a temperature not greater than 40° C.under reduced pressure. The solution thus obtained was filtered througha 0.05-μm filter to obtain Coating solution 1.

Synthesis Example 2

As in Synthesis Example 1, a mixture of 8.26 g of a 25% aqueous solutionof tetramethylammonium hydroxide, 34.97 g of ultrapure water, and 376.80g of ethanol was heated to 60° C. in advance. A mixture of 17.05 g oftetramethoxysilane and 15.26 g of methyltrimethoxysilane was addeddropwise over 1 hour, followed by the dropwise addition of a mixture of6.49 g of 1,2-bis(trimethoxysilyl)ethane and 6.54 g ofmethyltrimethoxysilane over 15 minutes. Neutralization, concentration,washing with water, re-concentration, and filtration were performed in asimilar manner to those of Synthesis Example 1 to obtain Coatingsolution 2.

Synthesis Example 3

As in Synthesis Example 1, a mixture of 8.26 g of a 25% aqueous solutionof tetramethylammonium hydroxide, 34.97 g of ultrapure water, and 376.80g of ethanol was heated to 60° C. in advance. A mixture of 21.92 g oftetramethoxysilane and 19.62 g of methyltrimethoxysilane was addeddropwise over 1 hour, followed by the dropwise addition of a mixture of2.16 g of 1,2-bis(trimethoxysilyl)ethane and 2.20 g ofmethyltrimethoxysilane over 15 minutes. Neutralization, concentration,washing with water, re-concentration, and filtration were performed in asimilar manner to those of Synthesis Example 1 to obtain Coatingsolution 3.

Synthesis Example 4

As in Synthesis Example 1, a mixture of 8.26 g of a 25% aqueous solutionof tetramethylammonium hydroxide, 34.97 g of ultrapure water, and 376.80g of ethanol was heated to 60° C. in advance. A mixture of 19.48 g oftetramethoxysilane and 17.44 g of methyltrimethoxysilane was addeddropwise over one hour, followed by the dropwise addition of a mixtureof 5.10 g of 1,4-bis(trimethoxysilyl)benzene and 4.36 g ofmethyltrimethoxysilane over 15 minutes. Neutralization, concentration,washing with water, re-concentration, and filtration were performed in asimilar manner to those of Synthesis Example 1 to obtain Coatingsolution 4.

Synthesis Example 5

As in Synthesis Example 1, a mixture of 8.26 g of a 25% aqueous solutionof tetramethylammonium hydroxide, 34.97 g of ultrapure water, and 376.80g of ethanol was heated to 60° C. in advance. A mixture of 19.48 g oftetramethoxysilane and 17.44 g of methyltrimethoxysilane was addeddropwise over one hour, followed by the dropwise addition of a mixtureof 4.10 g of 1,4-bis(trimethoxysilyl)methane and 4.36 g ofmethyltrimethoxysilane over 15 minutes. Neutralization, concentration,washing with water, re-concentration, and filtration were then performedin a similar manner to those of Synthesis Example 1 to obtain Coatingsolution 5.

Synthesis Example 6 Silicon Oxide Derivative Obtained by IntermediateAging after Preparation of a Core

As in Synthesis Example 1, a mixture of 8.26 g of a 25% aqueous solutionof tetramethylammonium hydroxide, 34.97 g of ultrapure water, and 376.80g of ethanol was heated to 60° C. in advance. A mixture of 19.48 g oftetramethoxysilane and 17.44 g of methyltrimethoxysilane was addeddropwise over one hour. After completion of the dropwise addition, thereaction mixture was aged for one hour without changing the temperature.Then, a mixture of 4.33 g of 1,2-bis(trimethoxysilyl)ethane and 4.36 gof methyltrimethoxysilane was added dropwise over 15 minutes.Neutralization, concentration, washing with water, re-concentration, andfiltration were performed in a similar manner to those of SynthesisExample 1 to obtain Coating solution 6.

Synthesis Example 7 Silicon Oxide Derivative Having an IntermediateLayer)

As in Synthesis Example 1, a mixture of 8.26 g of a 25% aqueous solutionof tetramethylammonium hydroxide, 34.97 g of ultrapure water, and 376.80g of ethanol was heated to 60° C. in advance. A mixture of 17.05 g oftetramethoxysilane and 15.26 g of methyltrimethoxysilane was addeddropwise over 60 minutes. Forty five minutes after the dropwise additionwas started, the dropwise addition rate was reduced to half and at thesame time, the dropwise addition of a mixture of 6.49 g of1,2-bis(trimethoxysilyl)ethane and 6.54 g of methyltrimethoxysilane wasstarted. When the dropwise addition of teramethoxysilane andmethyltrimethoxysilane was completed after 15 minutes, the dropwiseaddition rate was doubled and dropwise addition of the latter mixturewas performed over 30 minutes in total. Neutralization, concentration,washing with water, re-concentration, and filtration were performed in asimilar manner to those of Synthesis Example 1 to obtain Coatingsolution 7.

Comparative Synthesis Example 1

As in Synthesis Example 1, a mixture of 8.26 g of a 25% aqueous solutionof tetramethylammonium hydroxide, 34.97 g of ultrapure water, and 376.80g of ethanol was heated to 60° C. in advance. A mixture of 24.36 g oftetramethoxysilane and 21.80 g of methyltrimethoxysilane was addeddropwise over 1 hour. Neutralization, concentration, washing with water,re-concentration, and filtration were performed in a similar manner tothose of Synthesis Example 1 to obtain Coating solution 8.

Comparative Synthesis Example 2

As in Synthesis Example 1, a mixture of 8.26 g of a 25% aqueous solutionof tetramethylammonium hydroxide, 34.97 g of ultrapure water, and 376.80g of ethanol was heated to 60° C. in advance. A mixture of 21.63 g of1,2-bis(trimethoxysilyl)ethane and 21.80 g of methyltrimethoxysilane wasadded dropwise over 1 hour. Neutralization, concentration, washing withwater, re-concentration, and filtration were performed in a similarmanner to those of Synthesis Example 1 to obtain Coating solution 9.

Comparative Synthesis Example 3

A mixture of 8.26 g of a 25% aqueous solution of tetramethylammoniumhydroxide, 34.97 g of ultrapure water, and 376.80 g of ethanol washeated to 60° C. in advance. A mixture of 17.26 g of1,2-bis(trimethoxysilyl)ethane and 17.39 g of methyltrimethoxysilane wasadded dropwise over 1 hour, followed by the dropwise addition of amixture of 4.86 g of tetramethoxysilane and 4.35 g ofmethyltrimethoxysilane over 15 minutes. After completion of the dropwiseaddition, the reaction mixture was cooled to 40° C. or less andneutralized with an aqueous solution of maleic acid. After addition of150 g of propylene glycol propyl ether, the resulting mixture wasconcentrated at a temperature not greater than 40° C. under reducedpressure to distill off ethanol. To the residue was added 300 ml ofethyl acetate, followed by washing three times with 200 ml of ultrapurewater. Then, 200 ml of propylene glycol propyl ether was added and theresulting mixture was re-concentrated at a temperature not greater than40° C. under reduced pressure. The solution thus obtained was filteredthrough a 0.05-μm filter to obtain Coating solution 10.

Examples 1 to 7 and Comparative Examples 1 to 3

Each of Coating solutions 1 to 7 (Examples 1 to 7) and Coating solutions8 to 10 (Comparative Examples 1 to 3) was applied onto a Si wafer byspin coating. After soft baking at 120° C. for 2 minutes and at 200° C.for 2 minutes, the resulting wafer was baked at 400° C. for 1 hour in abaking furnace.

The dielectric constant of the porous film thus obtained was measuredbefore washing (initial) and after washing of the porous film. Thewashing treatment of the porous film was performed by dipping the porousfilm in “EKC-520” (trade name; product of Dupont) at room temperaturefor 10 minutes. The dielectric constant was measured with “495-CVSystem” (trade name; product of SSM Japan). The elastic modulus(modulus) was measured using a nanoindenter (product of NanoInstruments). The measurement results of Examples 1 to 7 and ComparativeExamples 1 to 3 are shown in Table 1.

TABLE 1 Initial Vlue Value After Washing Modulus Modulus K-value (GPa)K-value (GPa) Example 1 2.43 6.9 2.45 6.6 Example 2 2.39 6.6 2.41 6.4Example 3 2.48 7.0 2.52 6.7 Example 4 2.41 6.7 2.43 6.5 Example 5 2.457.0 2.48 6.7 Example 6 2.28 5.8 2.32 5.6 Example 7 2.41 6.6 2.44 6.4Comparative Example 1 2.51 7.2 2.78 4.8 Comparative Example 2 2.29 3.42.3 3.4 Comparative Example 3 2.32 3.6 2.68 3.6

As shown in Table 1, the porous film of Comparative Example 1 preparedwithout forming a shell showed significant deterioration by the washingtreatment, while the porous film of Comparative Example 2 prepared usingonly the shell component showed a low modulus of elasticity. As isapparent from the initial values of physical properties, the porousfilms prepared in Example 1 to 7 have enhanced strength, reflecting thestrength of the core component. With regard to the properties after thetreatment of the washing fluid, deterioration of the porous films isvery small, reflecting the stability of the shell component.

Having thus described certain embodiments of the present invention, itis to be understood that the invention defined by the appended claims isnot to be limited by particular details set forth in the abovedescription as many apparent variations thereof are possible withoutdeparting from the spirit or scope thereof as hereinafter claimed. Thefollowing claims are provided to ensure that the present applicationmeets all statutory requirements as a priority application in alljurisdictions and shall not be construed as setting forth the full scopeof the present invention.

1. An organic silicon oxide fine particle comprising: a core containingat least an inorganic silicon oxide or an organic silicon oxide and ashell containing at least an organic silicon oxide and being formedaround the core by using shell-forming hydrolyzable silane in thepresence of a basic catalyst; wherein of silicon atoms constituting thecore or the shell, a ratio (T/Q) of a number (T) of silicon atoms havingat least one bond directly attached to a carbon atom to a number (Q) ofsilicon atoms having all of four bonds attached to an oxygen atom isgreater in the shell than in the core; and wherein the shell-forminghydrolyzable silane comprise at least a hydrolyzable silane compoundhaving two or more hydrolyzable-group-having silicon atoms bound to eachother via a carbon chain or via a carbon chain containing one siliconatom between some carbon atoms.
 2. The organic silicon oxide fineparticle according to claim 1, wherein said shell-forming hydrolyzablesilane comprise one or more compounds represented by the followingformula (1):{R¹ _(n)X¹ _(3-n)Si—[(Y²)—(SiR² _(m)X² _(2-m))]_(p)}_(q)—(Y³)—SiR³_(t)X³ _(3-t)  (1) wherein X¹ to X³ each independently represents ahydrolyzable group selected from the group consisting of a hydrogenatom, halogen atoms and C₁₋₄ alkoxy groups; R¹ to R³ each independentlyrepresents a C₁₋₂₀ alkyl group or a C₆₋₁₀ aryl group; Y² and Y³ eachindependently represents a substituted or unsubstituted C₁₋₆ hydrocarbongroup having q+1 valencies, a C₅₋₂₀ cycloalkane group which has q+1valencies and may contain a fused ring structure, or a C₆₋₂₀ aromaticgroup having q+1 valencies; m each independently represents an integerfrom 0 to 2; n each independently represents an integer from 0 to 2; peach independently represents an integer from 0 to 4; q eachindependently represents an integer of 1 or greater; and t eachindependently represents an integer from 0 to
 2. 3. The organic siliconoxide fine particle according to claim 2, wherein said one or morecompounds represented by the formula (1) are selected from the groupconsisting of compounds represented by the following formula (2):

and the following formula (3):

wherein X⁴ to X⁹ each independently represents a hydrolyzable groupselected from the group consisting of a hydrogen atom, halogen atoms andC₁₋₄ alkoxy groups; R⁴ to R⁹ each independently represents a C₁₋₂₀ alkylgroup or a C₆₋₁₀ aryl group; m each independently represents an integerfrom 0 to 2; n each independently represents an integer from 0 to 2; peach independently represents an integer from 0 to 4; r eachindependently represents an integer from 0 to 4; s each independentlyrepresents an integer from 0 to 4; t each independently represents aninteger from 0 to 2; and u each independently represents an integer from0 to
 4. 4. The organic silicon oxide fine particle according to claim 1,wherein the number of silicon atoms contained in the core is greaterthan the number of silicon atoms contained in the shell.
 5. The organicsilicon oxide fine particle according claim 1, wherein the core containsa zeolite-like recurring structure.
 6. The organic silicon oxide fineparticle according to claim 1, wherein said inorganic silicon oxide orsaid organic silicon oxide of said core is an inorganic or organicsilica prepared by hydrolysis and condensation of a core-forminghydrolyzable silane in the presence of a basic catalyst.
 7. The organicsilicon oxide fine particle according to claim 1, wherein saidshell-forming hydrolyzable silane consists essentially of one or morehydrolyzable silane compounds having a carbon atom directly attached toa silicon atom.
 8. The organic silicon oxide fine particle according toclaim 1, further comprising an intermediate layer between the core andthe shell.
 9. A method for producing an organic silicon oxide fineparticle, comprising steps of: adding first hydrolyzable silane to wateror a mixed solution of water and alcohol to carry out hydrolysis andcondensation of the resulting mixture in the presence of a basiccatalyst to form a core, wherein the first hydrolyzable silane is asilane compound or compounds, containing at least one compoundrepresented by the following formula (4):Si(OR¹⁰)₄  (4) wherein R¹⁰ may be the same or different and eachindependently represents a linear or branched C₁₋₄ alkyl group; andadding, to the reaction mixture for the core, second hydrolyzable silanewhich is a hydrolyzable silane compound or a mixture of two or morehydrolyzable silane compounds to form a shell, wherein of silicon atomsconstituting the first hydrolyzable silane or the second hydrolyzablesilane, a ratio (T/Q) of a number (T) of silicon atoms having at leastone bond directly attached to a carbon atom to a number (Q) of siliconatoms having all of four bonds attached to an oxygen atom is greater inthe second hydrolysable silane than in the first hydrolysable silane;and the second hydrolyzable silane contains a hydrolyzable silanecompound having two or more hydrolyzable-group-having silicon atomsbound to each other via a carbon chain or via a carbon chain containingone silicon atom between some carbon atoms.
 10. The method for producingan organic silicon oxide fine particle according to claim 9, whereinafter addition of a total amount of the first hydrolyzable silane,reaction conditions permitting progress of the hydrolysis andcondensation of the added first hydrolyzable silane are maintained andthen the step of adding of the second hydrolyzable silane is started.11. The method for producing an organic silicon oxide fine particleaccording to claim 9, wherein prior to completion of the addition of atotal amount of the first hydrolyzable silane, the step of adding of thesecond hydrolyzable silane is started.
 12. The method for producing anorganic silicon oxide fine particle according to claim 9, wherein thesecond hydrolyzable silane is represented by the following formula (1):{R¹ _(n)X¹ _(3-n)Si—[(Y²)—(SiR² _(m)X² _(2-m))]_(p)}_(q)—(Y³)—SiR³_(t)X³ _(3-t)  (1) wherein X¹ to X³ each independently represents ahydrolyzable group selected from the group consisting of a hydrogenatom, halogen atoms and C₁₋₄ alkoxy groups; R¹ to R³ each independentlyrepresents a C₁₋₂₀ alkyl group or a C₆₋₁₀ aryl group; Y² and Y³ eachindependently represents a substituted or unsubstituted C₁₋₆ hydrocarbongroup having q+1 valencies, a C₅₋₂₀ cycloalkane group which has q+1valencies and may contain a fused ring structure, or a C₆₋₂₀ aromaticgroup having q+1 valencies; m each independently represents an integerfrom 0 to 2; n each independently represents an integer from 0 to 2; peach independently represents an integer from 0 to 4; q eachindependently represents an integer of 1 or greater; and t eachindependently represents an integer from 0 to 2.